All other things being equal, the difference in power (110W vs. 100W = +0.41dB) will not make much difference in how loud your system will play. However all things are rarely equal. Some units are able to deliver their rated power when all channels are driven whereas other units are only able to deliver a fraction of their rated power when driving all the speakers. At this point, you do not know if your JVC is defective, the speakers are too demanding of a load or you are just trying to play it louder than it is capable of being played. Without a side by side comparison you will not be able to determine if the Pioneer will be an improvement.
If the dealer selling the VSX515k has a liberal return policy, the best course of action would to be to buy one, take it home and compare it to the JVC.